Tamoxifen dose response and conditional cell marking: Is there control?

Conditional marking and gene inactivation experiments are valuable approaches used to understand developmental and molecular mechanisms. CreERT is a fundamental component in recombinase-based conditional strategies and is used to gain temporal control subsequent to tamoxifen administration. We tested the hypothesis that tamoxifen dose linearly correlates with recombination efficiency in vivo. Wnt1-CreERT and tamoxifen administration were used to mark progenitors that contributed to the trigeminal ganglia. We executed a dose response study to determine the number of neurons that had undergone recombination in response to tamoxifen administered at doses ranging from 50 to 500 mg/kg. Our findings show a substantial variability in the amount of recombination within and between dose groups with no clear correlation between tamoxifen dose and the number of marked cells. This is the first study that we are aware of in which cell counts, robust quantitative data, and statistical analyses were performed on sections obtained from embryos marked in response to a wide range of tamoxifen dose in vivo. We provide an important quantitative and statistical framework for designing CreERT-based experiments and choosing tamoxifen dosing paradigms.